X-ray tubes



May 1959 SAMUEI; "r. YANAGISAWA 2,886,725

X-RAY TUBES 2 Sheets-Sheet 1 Filed July 22, 1955 y 12, 1959 SAMUEL 1'. YANAGISAWA 2,886,725 X-RAY TUBES Filed July 22, 1955 2 Sheets-Sheet 2 INVENTOR.

SAMUEL T. YANAGISAWA ATTOR Y Unite States Patent X-RAY TUBES Samuel T. Yanagisawa, Stamford, Conn., assignor to Macblett Laboratories, Incorporated, Springdale, Conn., a corporation of Connecticut Application July 22, 1955, Serial No. 523,712 12 Claims. Cl. 313-57 sistent with respect to both duration and intensity, and

produces an X-ray beam emanating from a relatively fine focal spot so as to produce a clearly defined image when used, for example, in high speed radiography.

Flash X-ray tubes constructed and operated in accordance with teachings of the prior art have not been entirely satisfactory, particularly because they failed after producing a relatively small number of short duration flashes. Some prior art flash X-ray tubes, for ex ample, were able to produce small focal spots and pulses of relatively short duration, but were deficient in that they had a short life consisting of approximately twenty flashes. Since these tubes were relatively expensive, short life was a serious problem.

The presently described tube consistently produces several hundred separate fractional microsecond pulses, each roughly of .1 to .2 of a microsecond in duration, without warm up periods, so that the device is ready for immediate use when desired as in the investigation of high speed phenomena such as explosive detonations, high speed jets in shaped charges, projectile action, shrapnel distribution, physical action of armor piercing projectiles, and in many other applications.

A major factor contributing to short life of flash X- ray tubes is the problem of vaporization of tungsten It also limits the duration of the discharge by lowering tube impedance and tube drop to such a point that the production of X-rays is terminated. V i

In the presently described tube design, the ionized vapor is constrained from depositing in large amounts upon any insulating surface by the inherent shielding effect of a novel coaxial type of construction and by a novel geometric relationship between the anode and cathode. The electrode structure is designed to focus the electronsemitted by the cathode onto the target of a conical anode and also shapes the electrical field in such a manner that most of the tungsten ions flow from the target directly back to the emissive area of the cathode. This is done by shaping the focusing surface of the cathode or anode in any one of several ways such as by providing the adjacent emitting surfaces of the cathode and trigger with bevels having surfaces lying substantially in a common plane parallel with the effective surface of the target, by providing the adjacent ends of the cathode and trigger with flat parallel surfaces which are located in stepped relation to form a focusing ring, or by providing the anode with a particular shape which will create the desired electrical field.

A further problem associated with many prior art flash tubes is that the trigger-cathode gap spacing is caused to be altered as a result of expansion of the caused by the tremendous rate of heat dissipation on the surface of a tungsten anode during the fractional microsecond of a tubes operation. These tungsten vapors generally accumulate on the inner walls and surfaces of the tube, including the window through which the X-rays pass. reduce the intensity of the X-ray beam. However, even more serious, the deposits of large amounts of vaporized tungsten on the inner insulating walls of the tube contribute to causing flashes of weak intensity and long duration. charge up and cause spontaneous or premature flashes and may also cause defocusing and misdirection of the electron beam. Deposits also shorten the leakage path between the anode and cathode to such an extent that Such deposits on the window of course parts during the exhaust cycle. The present invention embodies a tubular cathode member and a tubular-trigger located in coaxial, substantially end-to-end relation with the trigger being of slightly smaller outside diameter than the inside diameter of the cathode memher. When expansion occurs during the exhaust cycle, the cathode member and trigger are able to slide .or telescope relative to one another and to subsequently return to their normal spaced relation without damage to the parts or upset to the desired space dimensions.

Therefore, it is accordingly a primary object of this invention to provide a long lived flash X-ray tube capabio of producing a relatively large number of X-ray pulses of high intensity and of relatively constant fractional microsecond duration.

Another object is to provide a flash X ray tube wherein vapor ions emanating from the anode during operation-of the tube will be directed or attracted toward the vicinity of the cathode-trigger gap.

Another object is to provide a flash X-ray tube with a cathode of predetermined structural arrangement such that, upon operation of the tube, electrons emitted by the cathode are directed onto the efiective portion of the anode and resultant ions emitted by the anode are directed onto the cathode.

The deposits on insulating parts of the tube H the tube may flash along the wall between two electrodes, and may also form a conductive coating between the trigger and cathode electrodes so that a proper cathode arc can no longer be formed.

A distinctive feature of the present invention is that the electrode structure is arranged to direct the initial of a cathode-trigger electrode arrangement the parts of Another object is the provision of a tube of the above character wherein the transit time for passage of tungsten ions from the anode to the cathode is controlled so as to permit operation of the tube for a controlled length of time before the ionsarrive at the cathode in an amount suflicient to terminate the production of X- rays.

Another object is the provision of a flash X-ray tube wherein electrons are directed toward the active portion of an axially located conical anode by the effective emitting portion of a cylindrical cathode encircling the anode, and wherein a shaped electrical field is created for attracting vapor ions from the anode to the emitting portion of the cathode structure. 1

Another object is the provision in a flash. X-ray tube which are capable of relative sliding or "telescopicmotion when expanded by'heat during the tube exhaust cycle. 11

A (further object is to provide .a tube .theabove character with improved shielding means to'reduce d;-

posits of vaporized target material on the inner walls of I adjoining portions of the cathode and trigger electrodes shown in Fig. 1;

Figs. 3, 4 and 5 are enlarged fragmentary views partly in section showing three different arrangements of cathode and trigger electrodes; and

Figs. 6 .and 7 are enlarged fragmentary elevational views of the target portions of two anodes formed in accordance with this invention.

Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the several views, the tube shown in Fig. 1 includes a dielectric envelope portion 11) which is provided at one end with a reentrant portion 11 through which is an axial opening having sealed to its periphery one end of a metal sleeve 12. The inner end of the sleeve 12 is soldered to a .block 13 which is provided with an outward ly extending threaded end portion 14 to which is removably connected one end of a conductive member 15. The inner end of the block 13 carries one end of an anode supporting rod 16 which is preferably formed of molybdenum and which extends along the longitudinal axis of the tube toward the opposed end of the envelope. An anode 17 is secured to the free end of the rod 16 and comprises a cylinder of tungsten or like metal suitable for the purpose having a conical end portion 18 forming an elfective target.

The end of the envelope portion opposite the re entrant portion 11 has sealed thereto a cylindrical metal sleeve 19.. A cathode-supporting ring 21 has one end of second and third metal sleeves 21 and 22 scaled to opposite .sides thereof, sleeves 21 and 22 being sealed to the ring 21) as part of a preassembled cathode structure which is subsequently vacuum sealed to the envelope by joining sleeve 21 to sleeve 19. Sleeve 22 is likewise sealed to one end. of a second dielectric envelope portion 23 having sealed to its other end a fourth sleeve 24. A trigger electrode terminal is connected with sleeve 24 and consists of an apertured metal end portion 25, formed preferably of copper, which has a fifth sleeve 26 joinedv to it, which sleeve 26 is Vacuum sealed to sleeve 24 to complete the envelope assembly.

Within the aperture of end portion 25 is located an X- ray window '27 preferably of beryllium. The window 27 is vacuum sealed to its supporting structure by any suitable means well known in the art and is provided so as to permit exit of X-rays from the interior of the tube.

A generally tubular cathode 28 is secured at one end .to the inner side of the cathode-supporting ring 20 and extends axially toward the end portion 25. The extreme end of the cathode 28 is provided with an annular effective portion 29 which is formed with an inner surface 30 which is inclined with respect to the axis of the tube and to the walls of the cathode. The annular effective portion '29 of the cathode 28 may be formed as a separate annular member bonded to the end of the cathode 28 or may be an integral part of the cathode. The cathode 28, including portion 2?, is preferably but not necessarily formed of nickel or molybdenum or a combination thereof.

Fixedly attached at one end to the inner surface of the tube end portion 25 is a tubular trigger electrode 31 havingan outside diameter slightly smaller than the inside diameter of the cathode 28. The other end of the trigger terminates adjacent the end of the cathode 28 in a beveled surface 32 adapted to be located substantially in a common plane with the inclined surface 30 of the cathode portion 29 (Figs. 1 and 2). This construction permits the trigger 31 and cathode 28 both to expand when heated, as during the baking process of the exhaust cycle. In such an event the trigger 31 will slide or telescope within the cathode 28 without damage to the adjacent sharp field emission edges 33 and 34 (Fig. 2) or upset to the normal predetermined gap spacing between the trigger and cathode. With this construction the trigger and cathode can be so located that the gap 35 between the field emission edges 33 and 34 can be maintained relatively small, preferably about .010 inch.

It is to be understood here that the cathode 28 and trigger 31 may be in reversed positions if desired.

A relatively large diameter cathode is required for the tube of the present invention, in the neighborhood of 2 /2 in inside diameter for example, since I have found that this aids in the production of desired intensity and hardness of the resultant X-ray flash to be formed by the tube. If we review the normal action of a flash X-ray tube we find that when suitable voltage is applied to th tube, as described hereinafter, an arc is created at the trigger-cathode gap and electrons that are pulled out of the trigger-cathode arc are accelerated toward the anode, and at the anode they produce heat which, in turn, vaporizes the anode surface and, as a result, ions are formed. Ions in the metal vapors are then accelerated back to the trigger cathode gap in accordance with this invention. It takes a certain amount of time to accomplish each of these steps. However, the time period that is most significant is the ion transit time or the time that it takes tungsten ions to get from the anode to the cathode. If we look at the time periods involved in other steps of this process, we find that it takes an extremely short time for electrons to get from the cathode-trigger arc to the anode, which time may be in the order of 10- seconds. It takes an extremely short time for these electrons to heat the anode surface to a temperature at which it will vaporize, which time may be in the order of 10' seconds. Then some undetermined extremely short time period is needed for this vapor to ionize. Finally there is the transit time necessary to accelerate the heavy ions of tungsten from the anode back to the cathode. This transit time is dependent upon the number of charges on the ions, and is further dependent upon the tube voltage, and the spacing between the electrodes. Assuming that the ions are singly charged in the present tube, this transverse interelectrode spacing between the anode and cathode at 120 kv. must be in the order of about 1%". For other voltages, this spacing will vary. For lower voltages the spacing should be smaller, but for a general purpose tube it is believed that a cathode diameter of about 2 /2" is desirable. The cathode should be at least 1 /2 in diameter in order to function as desired and this is believed important because the transit time is in the order of magnitude of the duration of the X-ray flash as is shown by the fact that a cloud of tungsten ions arriving at the cathode from the anode terminates the production of X-rays. The space relation of the gap 35 to the anode 17 is, therefore, of considerable importance.

It is also necessary that electrons emitted by the cathode in the vicinity of the gap 35 be directed onto the conical surface 18 of the anode, which conical surface 18 should, therefore, be so located that it will receive the electron bombardment from the cathode.

However, the cathode structure may take any desired form, such as the inclined surfaces 30 and 32, which will accomplish the objective of not only focusing electrons on the conical portion 18 of the anode but also create an electrical field which will draw toward it ions emitted by the anode when the tungsten or other material used for the anode is vaporized. The cathode 28, and particularly portion 29, as well as the trigger 31, are accordingly shaped to create a field which will attract the vapor, or

vents stray electrons from bombarding the seal. shield 41 also reduces the high electric field present at more particularly the ions thereof, to the gap area of the cathode structure rather than allowing it to flow onto insulating walls of the tube and to accumulate thereon with undesirable effects.

In the operation of the tube, a positive high voltage is impressed upon the anode 17 from a suitable source by means such, for example, as a charged capacitor system (not shown) as is well known in the art. This high voltage is transmitted to the anode 17 through conductive member 115, block 13 and supporting rod 16.

Cathode supporting ring 20 also functions as a terminal which may be connected to a pulse transformer (not shown) in a manner similar to that shown and described in United States Patent No. 2,311,705 to C. M. Slack, and the tube metal end portion 25 likewise is connected to the transformer. With the high voltage on the anode 17, a pulse is applied across the cathode gap 35 between the field emission edges 33-34 at a predetermined instant. The resultant spark across the gap 35 developed between the field emission edges 33-34 causes the cathode to emit electrons which are directed toward the conical target portion 18 of the anode. 'Ihe charged capacitor system then discharges along the rods 15 and 16, anode 17, and through cathode-anode interelectrode space to the cathode at ground potential. The target is heated by the high voltage discharge and, when bombarded by the electrons from the cathode, emits X-rays which pass out of the tube through window 27.

During the high voltage discharge and the electron bombardment of the anode, the tungsten material of the target 18 is heated to a temperature above its melting point, vaporizing a portion of the tungsten. Such vapor normally would become deposited on the inner walls of the insulating portions 10, 11 and 23 of the envelope to eventually function detrimentally to the successful operation of the tube. However, to prevent this condition, the cathode structure is designed to attract the vapor ions in the manner described hereinbefore. In this way the amount of vapor escaping into other portions of the tube is greatly reduced. A tubular shield 49 (Fig. 1) is mounted on the cathode-supporting ring 20 and extends axially of the tube toward the reentrant portion 11 to further prevent accumulation of tungsten deposits on the walls of the tube. This causes most of the relatively small amounts of vaporized tungsten which escape from the attraction of the cathode field to be deposited on the walls of the shield.

Mounted on the block 13 and covering the metal to glass seal between the envelope reentrant portion 11 and sleeve 12 is an inverted cup-like shield 41 which pre-- The the seal.

While a specific cathode shape has been shown and described for creating a field which will attract vapor ions to the vicinity of the cathode-trigger gap, other examples of various cathode shapes suitable for this purpose are shown in Figs. 3, 4 and 5.

In Fig. 3 is shown a cathode structure which embodiesa tubular cathode 23a which has an integral effective end portion 29a with an inclined surface 30a and a trigger 31a similar to that shown in Figs. 1 and 2. The relation of the cathode structure to the anode 17a, or more particularly to the effective conical target portion 18a;

thereof is clearly apparent.

In Fig. 4 a satisfactorily designed cathode structure is: shown which embodies a cathode 28b and a trigger 31b which have flat adjacent ends 30b and 32b respective-- ly lying in planes normal to the axis of the tube rather than inclined as in Fig. 3.

The structure in Fig. 5 embodies a cathode 28c which has an end portion 290 which is shaped as an inwardly turned flange 36. The inner end 30c of the flange 29c and the adjacent end 320 of the trigger 31c are flat and at right angles to each other. Near the end of'the trigger 31c on the inner side thereof is an apertured disc 37 which functions to aid in focusing the electrons to the anode and attracting the vapor ions from the anode.

The anode may also be shaped to assist in the function of directing the vapor ions toward the cathode as shown in Fig. 6. In this structure the anode 17a is provided with a circumferential ledge or shelf 38 encircling the large end of the conical portion 18d. Another anode structure is shown in Fig. 7 wherein the small end of the conical portion 182 of the anode 1742 is provided with an axial protuberance 39.

From the foregoing description it is apparent that an improved flash X-ray tube has been produced which has a long life and is capable of generating a large number of X-ray pulses, each of high intensity and fractional microsecond duration, in accordance with the objects of this invention.

While the novel features of the invention have been shown and are pointed out in the annexed claims, it is to be understood that various omissions, substitutions and changes in the construction and arrangement of parts shown and described may be made by those skilled in the art without departing from the spirit of the invention. Therefore, it is to be understood that all matter shown or described is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. A flash X-ray tube comprising a vacuum sealed envelope, an anode electrode structure within. the envelope and embodying an anode disposed substantially on the axis of the tube, and an annular cathode electrode structure within the envelope in spaced encircling relation to the anode and comprising a tubular cathode member and a coaxial tubular trigger disposed in spaced end-to-end relation to provide a gap between their adjacent ends, said trigger and cathode member each being independently supported at a point remote from said adjacent ends and being of different diametric dimensions whereby theymay telescope when heated and expanded.

2. A flash X-ray tube comprising a vacuum sealed envelope, an anode supported within the envelope on one end thereof, an annular cathode structure within the envelope in predetermined spaced encircling relation to the anode and comprising a tubular member supported at one end by the envelope and having at its other end an active cathode portion, and an annular trigger mounted on the end wall of the envelope opposite that end which sup ports the anode, the trigger being coaxial with and closely adjacent to the active cathode portion and having an outside diameter slightly smaller than the inside diameter of the active cathode portion whereby the trigger and active cathode portion may telescope under thermal expansion stresses.

3. A flash X'ray tube comprising a substantially cylindrical envelope embodying a hollow dielectric shell sealed at one end, an annular terminal sealed at one end .to the other end of the shell, an annular dielectric portion sealed at one end to the other end of the annular terminal, a metal end wall positioned opposite the other end of the annular dielectric portion, and a substantially tubular seal connecting the end wall to the annular dielectric portion, an annular cathode mounted in the annularwterminal and having a shielding portion extending freely longitudially within the annular dielectric portion and terminating within the tubular seal, and an annular trigger having one end terminating adjacent the free end of the cathode and providing therewith an electron emitting area, the adjacent ends of the cathode and trigger being .of different diameters whereby theymay telescope when heated and expanded during operation of the tube, the trigger further having a tubular shielding portion extending toward and mounted on said end wall, the shielding portions of the trigger and cathode completely isolating dielectric -7 portions of the envelope from the electron emitting area.

4. A flash X-ray tube comprising a vacuum sealed envelope, a conducting rod supported at one end within the envelope and extending along the longitudinal axis of the envelope, an anode supported on the free end of the conducting rod and having a comically-shaped target thereon, an annular cathode terminal mounted in and forming a circumferential portion of the envelope, an annular coaxial cathode structure positioned within the envelope in predetermined spaced encircling relation to the anode and having an emitting portion for bombarding the target with electrons, said cathode structure comprising a tubular member supported at one end by the annular terminal and having at its other end an active cathode portion, and a tubular trigger mounted on an end wall of the envelope and extending coaxially toward the active cathode portion and in close predetermined spaced relation therewith, the trigger having an outside diameter slightly smaller than the inside diameter of the active cathode portion whereby they may telescope when heated and expanded.

5. A flash X-ray tube comprising a vacuum sealed envelopc, a conducting rod supported at one end within the envelope and extending along the longitudinal axis of the envelope, an anode supported on the free end of the conducting rod and having a conically-shaped target thereon, an annular cathode terminal mounted in and forming a circumferential portion of the envelope, an annular coaxial cathode structure positioned within the envelope in predetermined spaced encircling relation to the anode and having an emitting portion for bombarding the target with electrons, said cathode structure comprising a tubular member supported at one end by the terminal and having at its other end an active cathode portion, and a tubular trigger mounted on an end wall of the envelope and ex- 1tending coaxially toward the active cathode portion and in close predetermined spaced relation therewith, the trigger having an outside diameter slightly smaller than the inside diameter of the active cathode portion whereby they may telescope when heated and expanded, the active cathode portion and adjacent portion of the trigger embodying means for attracting vapor ions from the target to the vicinity of the emitting portion upon bombardment and subsequent heating and vaporizing of the surface of the target.

6. A flash X-ray tube comprising a vacuum sealed envelope, a conducting rod supported at one end within the envelope and extending substantially along the longitudinal axis of the envelope, an anode supported on the free end of the conducting rod and having a comically-shaped target thereon formed of a material capable of copiously emitting X-rays when bombarded by electrons and simultaneously emitting vapor ions when heated by such bombardment to vaporizing temperature, an annular cathode 'terminal mounted in and forming a circumferential portion of the envelope, an annular coaxial cathode structure positioned within the envelope in encircling relation to the target and comprising a tubular member supported at one end by the annular terminal and having at its other end an active cathode portion, and a tubular trigger mounted on an end wall of the envelope and extending toward the active cathode portion and in predetermined spaced relation therewith to form therebetween a cathode-trigger gap of predetermined size, the cathode structure being adapted to emit electrons from the vicinity of the gap when suitable voltage is applied across the tube for bombarding the target, the trigger having an outer diameter slightly smaller than the inner diameter of the active cathode portion whereby they may telescope when heated and expanded, the active cathode portion and adjacent portion of the trigger embodying means for form- ,ing an electrical field which will attract the vapor ions from the target to the vicinity of the gap upon bombard- .ment and vaporizing of the anode, and the interelectrode space between the gap and the target being of a predetermined dimension in accordance with the voltage across *8 the tube and number of charges on the vapor ions whereby the transit time for passage of the vapor ions from the target to the vicinity of the gap is such that accumulation of ions at the gap will cause termination of the electron discharge at the conclusion of a predetermined time interval. t t

7. In a flash X-ray tube, an electrode structure comprising an anode, an annular cathode positioned in predetermined spaced encircling relation to the anode, and an annular trigger in spaced coaxial relation to the cathode providing therebetween a cathode-trigger gap for emission of electrons to the anode, the anode being adapted when bombarded by electrons to heat and form vapor ions, the inner surfaces of the cathode and trigger adjacent the gap lying substantially in a common plane angled with respect to the axis of the structure and forming an electrical field which will direct said vapor ions from the anode to the vicinity of the gap.

8. In a flash X-ray tube, an electrode structure comprising an anode, an annular cathode positioned in predetermined spaced encircling relation to the anode, and an annular trigger in spaced coaxial relation to the cathode providing therebetween a cathode-trigger gap for emission of electrons to the anode, the anode being adapted when bombarded by electrons to heat and form vapor ions, the adjacent end portions of the cathode and trigger being substantially tubular in shape with the outside diameter of the end portion of the trigger being slightly smaller than the inside diameter of the end portion of the cathode and the extreme ends of both of said portions being substantially flat and normally lying substantially in a common plane and thereby forming an electrical field which will direct said vapor ions from the anode to the vicinity of the gap.

9. In a flash X-ray tube, an electrode structure comprising an anode, an annular cathode positioned in predetermined spaced encircling relation to the anode, and an annular trigger in spaced coaxial relation to the cathode providing therebetween a cathode-trigger gap for emission of electrons to the anode, the anode being adapted when bombarded by electrons to heat and form vapor ions, the end portion of the cathode adjacent the trigger having an inwardly directed annular flange provided with a relatively flat end surface extending substantially coaxial of the structure, and the adjacent end portion of the trigger being substantially tubular and having an outside diameter slightly smaller than the diameter of the end surface of the flange, the trigger further having a substan tially flat end normally lying substantially in a common plane with the adjacent peripheral edge of the adjacent end of the end surface of the flange and having an inner annular member spaced slightly from the flat end thereof for forming an electrical field which will direct said vapor ions from the anode to the vicinity of the gap.

10. In a flash X-ray tube, an electrode structure com prising an anode, an annular cathode structure positioned 'in spaced encircling relation to the anode and embodying an annular electron emitting gap for emission of electrons to the anode, the anode being a rodlike member having a conical active end portion for receiving the electrons and adapted thereupon to heat and form vapor ions, the anode further having a relatively flat annular portion en circling the conical end portion at its junction with the major portion of the rodlike member, the flat annular portion lying in a plane transversely of the axis of the structure and forming an electrical field which will direct said vapor ions to the vicinity of the gap.

11. A flash X-ray tube substantially as set forth in claim 1 wherein the cathode structure embodies means for shaping an electrical field which will direct vapor ions rorn the anode to the vicinity of the gap during operation of the tube.

12. A flash X-ray tube substantially as set forth in claim 2 wherein the anode structure embodies means for shaping an electrical ,field which will direct vapor ions from the anode to the vicinity of the gap during operation of the tube.

References Cited in the file of this patent 10 Lilienfeld Feb. 8, 1927 Forde Mar. 6, 1934 Bouwers Sept. 25, 1934 Huppert et a1 Sept. 8, 1936 Criscuolo et a1. Oct. 11, 1955 

